1. Field of the Invention
The present invention relates generally to vacuum holddown devices, more specifically to a method and apparatus for a print media holddown using a vacuum force, and particularly to automatically adapting a holddown for various print media sizes used by a hard copy apparatus employing wet dye printing.
2. Description of Related Art
It is known to use a vacuum induced force to adhere a sheet of flexible material to a surface, for example, for holding a sheet of print media temporarily to a platen. [Hereinafter, "vacuum induced force" is also referred to as "vacuum induced flow," "vacuum flow," or more simply as just "vacuum" or "suction".] Such vacuum holddown systems are a relatively common, economical technology to implement commercially and can improve throughput specifications. For example, it is known to provide a rotating drum with holes through the surface wherein a vacuum through the drum cylinder provides a suction force at the holes in the drum surface. [The term "drum" as used hereinafter is intended to be synonymous with any curvilinear implementation incorporating the present invention; while the term "platen" can be defined as a flat holding surface, in hard copy technology it is also used for curvilinear surfaces, such as a common typewriter rubber roller; thus, for the purposes of the present application, "platen" is used generically for any shape paper holddown surface used in a hard copy apparatus.]
In a hard copy apparatus, such as a copier or a computer printer, a platen is used either to transport cut-sheet print media to an internal printing station or to hold the sheet media at the printing station while images are formed, or both. [In order to simplify discussion, the term "paper" is used hereinafter to refer to all types of print media; no limitation on the scope of the invention is intended nor should any be implied.] One universal problem is the management of different sized paper. Open holes around the edges of a sheet smaller than the dimensions of the vacuum field in the platen surface results in vacuum losses for holding the paper. In other words, too many exposed vacuum ports results in a change of the flow forces in each vacuum port and a loss of holding pressure at covered ports. Thus, a sheet of paper that is smaller than the total vacuum field is not firmly adhered to the surface. Known apparatus generally rely on a user manually switching operational functions to adjust the vacuum field to match the size of the paper in current use.
Another problem has become evident as attempts have been made to employ vacuum for holding paper in "wet" printing environments, that is, in hard copy apparatus such as in an ink-jet printer that uses a liquid dye. [The terms "liquid dye," or "wet dye" or just "dye" is used herein as generic for all such hard copy apparatus, whether employing ink (which may itself be dye-based or pigment-based), a wet toner, or other liquid colorant.] The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
For example, with a drum surface employing a field of discrete vacuum holes, the localized vacuum pressure against the underside of the paper draws the wet dye through the capillaries of the paper material before the dye has time to set. This results in alternating dark and light concentrations of dye in the final image correlating to the individual influence regions of the holes in the field. Moreover, in an ink-jet environment, air flow due to vacuum forces through ports around the periphery of the paper could affect ink drop firing trajectories, resulting in misprints or random artifacts in the final image.
Another problem occurs in ink-jet printing when the pen-to-paper spacing varies across the surface of the paper. If this spacing variation is rapid, print defects occur due to droplet trajectory errors and flight time differences. Such spacing variation occurs if the paper is locally deformed by vacuum ports of significant size, e.g., greater than about one to two millimeters.
There is a need for a vacuum holddown that can automatically adjust to a relatively universal variety of sizes of a flexible material. The holddown system should operate while being moved at a relatively high speed (e.g., for a drum rotating at approximately 30-inches/second). Moreover, there is a need for a vacuum paper holddown that is suited for use in a wet dye printing environment.